1 The construction of a raised dike across the coastal saltmarshes of Odiel (Huelva, south-west Spain) in 1977 divided a uniform area of low-lying sediment into two lagoons with very different drainage properties. To the east of the dike, there is rapid drainage into the estuarine channel of the Odiel and Tinto rivers, whereas to the west, the development of sand spits has impeded drainage such that standing water persists long after high tides, despite the slightly higher elevation of this area. 2 Both lagoons have been colonized by isolated clones of Spartina maritima, which have locally enhanced accretion to form domed tussocks (`nucleation'). The diameter of the tussocks was highly correlated with the elevation of their sediment surface, the larger tussocks having been the earlier colonists in a generally accreting system. Spartina in the interior of the tussocks showed reduced tiller density and vigour. 3 Only in the better-drained lagoon have the central, higher areas of the Spartina tussocks been invaded by Arthrocnemum perenne, although seed is freely available in both lagoons. Arthrocnemum formed a sprawling, dense canopy and a superficial, relatively impenetrable root system; it rapidly suppressed the remaining tillers of Spartina, eventually leaving only a fringe around the edge of the tussock. Areas invaded by Arthrocnemum were characterized by a superficial layer (10 cm) of oxidizing sediment $(E_h > + 80 mV)$. Spartina-dominated areas in both lagoons remained highly reducing, even in the surface layers ($E_h < - 150 mV$). 4 In terms of successional mechanisms, primary colonization by Spartina maritima facilitates the invasion by Arthrocnemum, which only becomes established from seed on relatively well-drained, oxidising sediments. The interaction is, however, more complicated than this: declining tiller density within the tussocks prior to invasion by Arthrocnemum is consistent with an inhibition mechanism, and the superior competitive ability of Arthrocnemum, once established, suggests that a tolerance mechanism may also operate

Summary1. The scarcity of reliable long-term phenological data has severely hindered the study of the responses of species to climate change. Biological collections in herbaria and museums are potential sources of long-term data for such study, but their use for this purpose needs independent validation. Here we report a rigorous test of the validity of using herbarium specimens for phenological studies, by comparing relationships between climate and time of peak flowering derived from herbarium records and from direct field-based observations, for the terrestrial orchid Ophrys sphegodes. 2. We examined herbarium specimens of O. sphegodes collected between 1848 and 1958, and recorded peak flowering time directly in one population of O. sphegodes between 1975 and 2006. The response of flowering time to variation in mean spring temperature (March-May) was virtually identical in both sets of data, even though they covered different periods of time which differ in extent of anthropogenic temperature change. In both cases flowering was advanced by c. 6 days per°C rise in average spring temperature. 3. The proportion of variation in flowering time explained by spring temperature was lower in the herbarium record than in direct field observations. It is likely that some of the additional variation was due to geographical variation in collection site, as flowering was significantly earlier at more westerly sites, which have had warmer springs, over their range of 3.44°of longitude. 4. Predictions of peak flowering time based on the herbarium data corresponded closely with observed peak flowering times in the field, indicating that flowering response to temperature had not altered between the two separate periods over which the herbarium and field data were collected. 5. Synthesis. These results provide the first direct validation of the use of herbarium collections to examine the relationships between phenology and climate when field-based observational data are not available.

Summary1 Submerged plants in shallow lakes are subject to pulling forces arising from waves, currents and grazing birds. Such forces can cause anchorage failure (mainly dislodgement of the root system) or breaking failure of the stems. Both lead to loss of fitness but uprooting is more damaging because many perennial species can replace broken shoot systems. 2 We investigated 12 abundant species ( Ceratophyllum demersum , Chara sp., Eleogiton fluitans , Elodea canadensis , Myriophyllum spicatum , Najas marina , Potamogeton natans , P. obtusifolius , P. pectinatus , P. pusillus , Utricularia vulgaris and Zannichellia palustris ) in 28 shallow lakes in the UK and the Netherlands. We measured the anchorage and breaking strengths of individual plants of different sizes. 3 Anchorage strength depends on the cohesive strength of the sediment and the size of the root system. The undrained shear-strength of sediments in shallow lakes varied more than 50-fold, but all were substantially weaker than terrestrial soils. Anchorage strength was modelled using the product of sediment cohesive strength and four measures of root-system size. A transformation of plan-form area (raising it to the power 2/3) that represented the hemispherical surface area of the root ball was consistently the best predictor of anchorage strength. 4 Breaking strength was a linear function of stem cross-sectional area in all species. Breaking stresses were comparable with those of marine algae and non-lignified terrestrial plants. 5 The results were used, in combination with plant allometric relationships, to predict the fates of four of the species when challenged with the largest waves likely to be encountered in a 10-year period, and the even greater forces exerted by grazing birds. We show that sediment strength and plant size determine whether plants break or uproot. A careful balance between investment in anchorage and in breakage resistance is needed to survive in the fluctuating physical environment of lakes. 6 Pulling forces experienced by aquatic plants are distinct from the mainly bending forces on more rigid land plants. We provide the first theoretical and quantitative framework for understanding their effects. Anchorage failure associated with the soft sediments of eutrophic lakes is likely to be a factor in the loss of macrophyte communities and an important factor in their restoration.

Summary1. Coastal saltmarshes provide distinctive biodiversity and important ecosystem services, including coastal defence, supporting fisheries and nutrient cycling. However, c. 50% of the world's coastal marshes are degraded or have been lost, with losses continuing. In both Europe and North America, there is a legal requirement to create habitats to substitute for losses. How well do created habitats replicate natural salt marshes? 2. We compared plant communities and environmental characteristics of 18 deliberately realigned (managed realignment, MR -between 1 and 14 years old), 17 accidentally realigned (AR, 25-131 years old) sites with those on 34 natural reference saltmarshes in the UK. 3. Halophytic species colonized individual realignment sites rapidly, attaining species richness similar to nearby reference marshes after 1 year. Nevertheless, the community composition of MR sites was significantly different from reference sites, with early-successional species remaining dominant, even on the high marsh. 4. The dominance of pioneer species on the low and mid-marsh may be because, at the same elevation, sediments were less oxygenated than on reference sites. Sediments were well oxygenated on the high marsh, but were often drier than on natural marshes. 5. Overall community composition of AR marshes was not significantly different to reference marshes, but the characteristic perennials Limonium vulgare, Triglochin maritima, Plantago maritima and Armeria maritima remained relatively rare. In contrast, the shrub Atriplex portulacoides was more abundant, and its growth form may inhibit or delay colonization by other species. 6. Synthesis and applications. Marshes created by managed realignment do not satisfy the requirements of the EU Habitats Directive. Adherence to the Directive might be improved by additional management interventions, such as manipulation of topographic heterogeneity or planting of mid-and upper-marsh species. However, given the inherent variation in natural saltmarshes and projected environmental change, policies that require exact equivalence at individual sites may be unachievable. More realistic goals might require minimum levels of a range of ecosystem functions on a broader scale, across catchments or regions.

Glacial refugia and postglacial migration are major factors responsible for the present patterns of genetic variation we see in natural populations. Traditionally postglacial history has been inferred from fossil data, but new molecular techniques permit historical information to be gleaned from present populations. The chloroplast tRNA(Leu1) intron contains regions which have been highly conserved over a billion years of chloroplast evolution. Surprisingly, in one of these regions which has remained invariant for all photosynthetic organisms so far studied, we have found intraspecific site polymorphism. This polymorphism occurs in two European oaks, Quercus robur and Q. petraea, indicating hybridisation and introgression between them. Two distinct chloroplast types occur and are distributed geographically as eastern and western forms suggesting that these oaks are each derived from at least two separate glacial refugia.